It is often desirable to fuse component sheets of a thermoplastic material such as sheets of polyethylene, polyvinyl chloride (PVC), or polyurethane to form larger sheets, to fabricate more-complicated shapes, or to make attachments. The desired fusion may occur in connection with the fabrication of air-supported building space enclosures, liquid containment tanks, building awnings, or any of numerous other generally similarly constructed products.
Thermoplastic sheet components are made from a number of different materials or combinations of materials. They also are available in various thicknesses, styles, and surface textures. Because of the diversity of such materials, the variety of forms into which they may be fabricated, and varying aesthetic and reliability requirements, many different manufacturing systems have been proposed and utilized for joining thermoplastic sheet components.
The most basic equipment for fusing thermoplastic sheets consists of a hand-held hot air blower device and a hand roller. An operator uses the blower to apply heat between the material plies to be fused as they are rolled together using the roller element. This system is regularly used throughout industry to make repairs because it is highly portable and is readily adaptable to small and irregular jobs. It is also used to fabricate complex shapes as one would find in articles of protective clothing or fuel-containment cells for race cars. Although such techniques have been used effectively, it is slow and its success is limited by the skill of the operator. Poor joinder seams can occur if the hot air is applied for too long or too short a period of time or if the roller element is not used properly or in a timely fashion.
Automated equipment for fusing thermoplastic sheets can be divided into two groups. One group can be referred to as comprising rotary machines (the other group is comprised of press-type machines), and in this rotary machine group either the machine travels along the fused seam regions of the sheets to be joined or the sheet seam regions are passed through the machine. As the machine and thermoplastic sheets are moved relative to each other a heat source such as a hot air blower, infrared radiator, or heated wedge is used to heat the sheet areas to be fused. The heat source is followed by a pressure roller or combination of pressure rollers and sometimes by pressure exerting belt surfaces. Such rollers or belt surfaces force the sheet seam areas together and allow them to fuse together.
Machines of the rotary type are particularly useful for assembling large membranes in factories as well as in the field and are used extensively in the roofing and water containment industries. Due however to distortion of the sheets introduced by the motion of the pressure rollers and also due to sheet seam shrinkage during cooling, seams made with this equipment are seldom adequate in applications requiring a high degree of aesthetics as in awning applications or requiring a very flat seam such as is used in sign facings. Also, these machines generally do not maintain the seam under pressure throughout the cooling process and such allows certain materials to generate gasses within their structure and often results in seams weakened by large quantities of contained gas bubbles.
The second group of thermoplastic sheet fusing machines (the press type) are usually stationary during operation, and the thermoplastic sheet seam components to be joined are placed in the machine in their desired position with respect to each other. The machine is then actuated and seam fusing takes place over an extended area at one time rather than over the area linearly with time.
One commonly used press type machine is the radio-frequency welder. This type of machine usually consists of a frame supporting two opposing dies or platens, one of which is relatively stationary and the other relatively movable. Thermoplastic sheets to be fused are placed between the dies/platens and pressed. The sheets are then heated by passing radio-frequency energy through the sheets using the die/platen elements as antennas. When adequate heat has been generated to fuse the sheets, the radio-frequency energy is withdrawn (stopped) and the fused area is allowed to cool. The dies or platens are afterwards separated and the fused sheets removed.
Radio-frequency welders produce highly reliable and aesthetically superior seams, largely because the seam remains pressed between the press platens throughout the heating and cooling cycles. This stabilizes the fused area preventing shrinkage and warpage as well as insuring a uniformly consistent seam surface texture. Although the machine is generally successful, it does however suffer from certain shortcomings. When a sealed seam is being made, the area between the machine platens or dies must be filled with a material of uniform dielectric constant value. If such is not the case, radio-frequency energy will tend to concentrate in areas of least dielectric resistance and cause overheating at those locations. Such makes it very difficult to fuse sheet pieces smaller than the platens, to do intermittent seals, or to incorporate any metallic items in or near the fused areas.
A second press-type machine is the hot platen welder which is made in a number of different configurations. Generally the hot platen welder utilizes one or two heated platens to both press the materials together and transfer heat to the area to be fused. Although hot platen welders are effective for heating thermoplastics they do not provide control of the cooling process. The result being that some materials fuse successfully while others suffer from shrinking or distortion. Further problems arise from the tendency of some materials to generate gas bubbles if not maintained under pressure when heated and cooled. This may cause a sponge-like texture within the fused area thus making the seam weak and unacceptable.
Although many machines are available commercially for fusing thermoplastic sheet materials, none has yet adequately addressed the problems involved in making highly reliable and aesthetically superior seals of varying size and shape. Nor is automatic equipment available to do complex fabrications, especially those incorporating varying numbers of plies.
There is therefore the need for a machine capable of applying even heating to thermoplastic sheets in complex product configurations of varying thicknesses, and to maintain those sheets under controlled pressure and in a stabilized condition throughout controlled heating and cooling cycles.
Other objects of the present invention will become apparent during a careful consideration of the descriptive materials and claims which follow.